Conductivity Behavior of Salt Deposits on the Surface of Engineered Barrier Materials for the Potential High-Level Nucle
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Conductivity Behavior of Salt Deposits on the Surface of Engineered Barrier Materials for the Potential High-Level Nuclear Waste Repository at Yucca Mountain, Nevada Lietai Yang, Miriam R. Juckett, and Roberto T. Pabalan Center for Nuclear Waste Regulatory Analyses, San Antonio, TX 78238–5166, U.S.A. ABSTRACT The electrical conductance or conductivity of three salt mixtures, Na-K-Cl-NO3, Ca-K-Cl and Ca-NaCl, were measured at 25, 50 and 70 °C [77, 122, and 158 °F] as a function of relative humidity (RH). Mutual deliquescence and efflorescence RH (MDRH and MERH) values were determined based on the conductivity measurements. It was found that the conductivity of the three salt mixtures started to increase at RH values that are approximately 40 % of their MDRH and increased by 1 to 2 orders of magnitude just before reaching the MDRH. At the MDRH, a significant increase in conductivity was observed. The MDRH and MERH for the Ca-K-Cl and Ca-Na-Cl mixtures were found to be approximately 15 % in the temperature range of 50 to 70 °C [122 to 158 °F]. The MDRH and MERH for the Na-K-Cl-NO3 system were found to be approximately 54 % at 50 °C [122 °F] and decreased significantly with an increase in temperature. INTRODUCTION The U.S. Department of Energy (DOE) plans to permanently store the nation's spent nuclear fuel and defense high-level wastes in a potential geologic repository at Yucca Mountain, Nevada. Evaporation of ground water and deposition of aerosols and dusts from ventilation air may lead to the accumulation of hygroscopic salts on the surface of drip shield, waste package, and ground support materials [1]. These engineered materials are expected to be dry during the pre-closure period because of the forced ventilation and for some time after closure of the repository due to the radioactive decay heat generated by the spent nuclear fuel [1]. After a certain period of time after closure, the temperature in the repository drift will decrease and the relative humidity (RH) will increase. When the RH in the drift environment reaches the mutual deliquescence RH (MDRH) of the hygroscopic salt mixture that may be present on the surface of the engineered materials, the salt will absorb moisture from the atmosphere and form potentially corrosive brines. Aqueous corrosion requires the presence of an electrolyte that is electrically conductive to ionic species. In our previous work [2], the conductivity behavior of two pure salts (KCl and NaNO3) at 30 °C [86 °F] and a few selected pure salts and mixtures of salts at 50 °C [122 °F] was measured during the increase of RH. It was shown that the conductivity of the salts was below the detection limit of the instrument at low RH, but the conductivity started to increase above the detection limit when the RH (%) of the experimental system reached about 15 to 25 below the deliquescence RH (DRH) of the salt or salt mixture [2]. When the DRH or the equilibrium RH was reached, the conductivity increased dramatically as an aqueous phase was formed. The increase in conductivity at
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